The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In some wireless communication systems, one or more communication devices employ multiple antennas. Accordingly, a communication channel between two such devices can be a multiple-input, multiple-output (MIMO) channel when both communication devices employ multiple antennas, a single-input, multiple-output (SIMO) channel when a transmitting device (“the transmitter”) employs a single transmit antenna and the receiving device (“the receiver”) employs multiple receive antennas, or a multiple-input, single-output (MISO) channel the transmitter employs multiple transmit antennas and the receiver employs a single receive antenna. Referring for simplicity to MIMO communication systems, transmission and reception properties in these systems can be improved by using each of the various transmit antennas to transmit the same signal while phasing (and amplifying) this signal as it is provided to the various transmit antennas to achieve beamforming or beamsteering. Generally speaking, beamforming or beamsteering creates a spatial gain pattern having one or more lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, while generally reducing the gain over that obtained by an omni-directional antenna in other directions. If the gain pattern is configured to produce a high gain lobe in the direction of each of the receiver antennas or in the direction of the receiver antennas in general, the MIMO system can obtain better transmission reliability between a particular transmitter and a particular receiver, over that obtained by single transmitter-antenna/receiver-antenna systems.
To conduct beamforming in the direction of a receiver, a transmitter generally utilizes a steering matrix determined based on specifics of the forward channel (i.e. the channel from the transmitter to the receiver) to condition the signals applied to various transmission antennas so as to produce the desired transmit gain pattern. In a technique known as explicit beamforming, to determine the specifics of the forward channel, such as the channel state information (CSI) or other measured description of the forward channel, the transmitter first sends training data to the receiver, which then determines or estimates forward channel specifics and/or a steering matrix that specifies beamsteering coefficients for steering signals from the transmitter in the direction of the receiver and then transmits this information back to the transmitter. In implicit beamforming, on the other hand, the transmitter determines specifics of the reverse channel (the channel from the receiver to the transmitter) based on training signals that the transmitter receives from the receiver and estimates the forward channel from the reverse channel by assuming channel reciprocity. However, radio frequency (RF) chain impairments in the form of gain/phase imbalances impair reciprocity between the forward and the reverse channel, and therefore, in implicit beamforming, the transmitter generally compensates for these RF chain imbalances in some manner (e.g., through calibration and/or correction) to achieve the desired transmission pattern.
Some explicit and implicit beamforming techniques as well as some calibration techniques are described in the IEEE 802.11n Draft Standard (2009), incorporated herein fully by reference.